Polypyridobisimidazole polymer is a rigid rod polymer. Fiber made from this polymer, one polymer composition of which is referred to as PIPD, known as the polymer used to make M5® fiber, is known to be useful in both cut and flame resistant protective apparel. Sec for example, PCT Application WO199902169 and WO2005002376. Rigid-rod polymer fibers having strong hydrogen bonds between polymer chains, e.g., polypyridobisimidazoles, have been described in U.S. Pat. No. 5,674,969 to Sikkema et al. An example of a polypyridobisimidazole includes poly(1,4-(2,5-dihydroxy)phenylene-2,6-pyrido[2,3-d:5,6-d ′]bisimidazole), which can be prepared by the condensation polymerization of tetraaminopyridine and 2,5-dihydroxyterephthalic acid in polyphosphoric acid. Sikkema describes that in making one- or two-dimensional objects, such as fibers, films, tapes, and the like, it is desired that polypyridobisimidazoles have a high molecular weight corresponding to a relative viscosity (“Vrel” or “ηrel”) of at least about 3.5, preferably at least about 5, and more particularly equal to or higher than about 10, when measured at a polymer concentration of 0.25 g/dl in methane sulfonic acid at 25° C. Sikkema also discloses that very good fiber spinning results are obtained with poly[pyridobisimidazole-2,6-diyl(2,5-dihydroxy-p-phenylene)] having relative viscosities greater than about 12, and that relative viscosities of over 50 (corresponding to inherent viscosities greater than about 15.6 dl/g) can be achieved.
Thermal and flame retardant protective apparel has been used by firefighters, emergency response personnel, members of the military and racing personnel as well as industrial workers to save lives and reduce injury due to fires and other thermal events. While polypyridobisimidazole fiber has excellent fire resistant properties, superior in many respects to most other fibers, it also has a high modulus of elasticity. There is a concern that such high modulus fibers may create fabrics that reflect the fibers, meaning that these fabrics also have a high modulus or are relatively stiff and uncomfortable to wear. However, there is a desire to incorporate the superior fire resistance of polypyridobisimidazole fibers into fabrics so that additional lives may be saved. What is needed, therefore, is a fabric comprising polypyridobisimidazole that is both comfortable and still provides adequate fire protective performance.